This invention relates generally to power generating fuel cells and is particularly directed to the fabrication of solid oxide fuel cells.
There are several types of fuel cells currently being studied as possible alternatives for converting coal derived fuels to electricity. The three primary types of fuel cells under study are the phosphoric acid (PAFC), molten carbonate (MCFC), and solid oxide fuel cell (SOFC). The most important single factor in assessing the viability of an alternative power generating concept is its overall capital and operational cost to the user compared to the cost of conventional power generating systems. A number of factors affecting the economics of power generation based upon the fuel cell need to be considered in evaluating the commercial viability of a given fuel cell approach. One of the primary factors to be considered is the capital cost of the power generating unit of the system. The present invention is directed to the fabrication of a low cost SOFC.
The modular SOFC system is considered to be one of the viable technologies for future commercial installations. In its simplest form, an SOFC modular system is comprised of an array of ceramic based fuel cell tubes connected in series and further includes parallel configurations having the necessary fuel and air manifolds, plenum exhaust outlets, electrical interconnects, etc. All such fuel cells further include cathode, electrolyte and anode layers which are deposited upon a porous support tube with an interconnecting strip to form series and parallel electrical connections.
Such cells or stacks of cells operate at high temperatures to directly convert chemical energy of a fuel into direct current electrical energy by electrochemical combustion. This type of fuel cell utilizes a natural or synthetic fuel gas such as those containing hydrogen, carbon monoxide, methane and an oxidant such as oxygen or air. A typical SOFC reacts hydrogen fuel with oxygen from air to produce electrical energy, water vapor and heat. Cell operating temperatures are typically in the range of from 700.degree. to 1100.degree. C. Each cell contains an electrolyte in solid form which serves to insulate the cathode and anode from one another with respect to electron flow, but permits oxygen ions to flow from the cathode to the anode. The hydrogen reaction on the anode with oxide ions generates water with the release of electrons; and the oxygen reaction on the cathode with the electrons effectively forms the oxide ions. Electrons flow from the anode through an appropriate external load to the cathode, and the circuit is closed internally by the transport of oxide ions through the electrolyte. A selected radial segment of the cathode, or air electrode, is covered by an interconnect material, or interconnector, for forming the aforementioned series and parallel electrical connections with adjacent SOFCs.
Present methods of fabricating SOFCs contemplate the use of chemical vapor deposition (CFD) and electrochemical vapor deposition (EDV) for depositing the electrolyte material as well as the interconnector material on an SOFC support tube which is porous and preferably comprised of calcia stabilized zirconia. This process is carried out by an SOF fabrication apparatus 10 such as illustrated in FIG. 1. The SOFC fabrication apparatus 10 includes a base 12, to an upper portion of which is attached in a sealed manner a cylindrical reactor tube 14. The base 12 includes a manifold 26. High temperature water vapors such as in the form of a water/hydrogen gas mixture are directed through a duct 20 within the base 12 to permit the water vapors to flow upward through the manifold 26. An upper portion of the manifold 26 includes a plurality of spaced ducts 28 which direct the water vapors upward. Disposed about each of the ducts 28 is a generally cylindrical support member 16 preferably comprised of alumina. At the upper end of each of the support members 16 is attached an SOFC support tube, or porous air electrode substrate, 18. The water vapor is directed over the inner surface of each of the SOFC support tubes 18 and exits the tube fabrication apparatus 10 via an exhaust port 24. Chloride vapors 22 are introduced into an upper portion of the reactor tube 14 and are directed downward over the outer surface of each of the support tubes 18. The chloride vapors also exit the tube fabrication apparatus 10 via the exhaust port 24. A portion of the downward directed vapors are electrochemically deposited on the outer surface of the support tube 18 to form the oxidized interconnector and electrolyte layers. The reactor tube 14 is comprised of mullite and is heated externally by a plurality of resistance heaters 30 such as silicon carbide glowbars.
The electrochemical vapor deposition of the SOFC's interconnector is carried out at approximately 1300.degree. C., while the electrochemical vapor deposition of the SOFC's electrolyte is carried out at approximately 1200.degree. C. The H.sub.2 O/H.sub.2 gas mixture is passed up the center of the pre-air electrode coated support tubes 18, and reacts at the outer surface of these tubes with metal chloride gas in a partial vacuum to produce an impervious oxide layer. Because both the interconnector and the electrolyte layers of the SOFC tube are not fully circumferential, Cr2O.sub.3 powder masking is necessary to blank off areas not requiring coating.
To extend the present SOFC tube fabrication apparatus which is capable of fabricating 12 SOFC support tubes per reaction to a 1000 support tube capability per reaction, which is believed to be necessary for the commercial viability of this approach, would require at least a 3 foot diameter mullite reactor tube heated externally by an array of heating elements in a design and at a power level, the feasibility of which is yet to be shown. Moreover, the capability of fabricating a 3 foot diameter, 8-10 foot long mullite reactor tube has also yet to be demonstrated and is of questionable feasibility from a commercial standpoint. Finally, external radiant heating of the SOFC support tubes is not only inefficient and costly, but gives rise to nonuniform heating of the SOFC support tubes resulting in the nonuniform and irregular deposition of air electrode and electrolyte layers thereon.
Use of the SOFC tube fabrication apparatus 10 of FIG. 1 requires the following sequential steps:
(1) EXTRUSION OF SUPPORT TUBE PA1 (2) SINTERING OF SUPPORT TUBE PA1 (3) SLURRY COATING OF THE AIR ELECTRODE PA1 (4) SINTERING OF AIR ELECTRODE PA1 (5) INTERCONNECTOR MASK PA1 (6) ELECTROCHEMICAL-VAPOR DEPOSITION OF THE INTERCONNECTOR PA1 (7) DEMASKING OF THE INTERCONNECTOR MASK PA1 (8) ELECTROLYTE MASK PA1 (9) ELECTROCHEMICAL-VAPOR DEPOSITION OF THE ELECTROLYTE PA1 (10) FUEL ELECTRODE MASK PA1 (11) SLURRY COATING OF THE FUEL ELECTRODE PA1 (12) SINTERING OF FUEL ELECTRODE
The prior art approach to SOFC fabrication therefore involves a double EVD coating process wherein first the interconnector and then the electrolyte is deposited upon the support tube. This double EVD coating process is difficult to carry out, has not yet been proven in the large scale, commercial fabrication of SOFCs, and is time consuming and thus expensive.
The present invention addresses and overcomes the aforementioned limitations of the prior art by providing for the low cost, efficient and fast deposition of uniform interconnect and electrolyte layers on the porous support tube of an SOFC. The process and apparatus of the present invention are readily adapted to simultaneous processing of large numbers of SOFC support tubes using currently available technology so as to substantially enhance the commercial attractiveness of SOFCs as power generators.